Project summary: Homologous recombination that takes place in prophase of meiosis I is required for reproduction in humans and other eukaryotes. Defects in meiotic homologous recombination in humans cause infertility, miscarriages, and Down and Turner syndromes. The human MLH1-MLH3 complex (MutL?) has been implicated in meiotic homologous recombination, but its function in this process has not been defined. In addition to having a key function in meiotic homologous recombination human MutL? has other functions that are poorly understood. One of these functions is required for triplet repeat DNA expansion, a process that causes several neurodegenerative diseases. A lack of information about the action of MutL? in meiotic recombination and triplet repeat DNA expansion is a major gap in our understanding of these key biological processes. Our preliminary data support the hypothesis that human MutL? functions as an ATP-dependent endonuclease in meiotic recombination and triplet repeat DNA expansion. The goal of this project is to investigate MutL? and its potential interactors and establish functional assays for future studies of MutL? and MutL?-dependent mechanisms. In Aim 1, we will study endonuclease and ATPase activities of MutL? in assays that will produce novel insights into the functions of MutL? in meiotic homologous recombination and triplet repeat instability. In Aim 2, we will identify proteins that interact with human MutL?. The results of the proposed research will advance our understanding of MutL? and will permit new studies into the mechanisms of MutL?-dependent meiotic recombination and triplet repeat DNA expansion.